Summary.Pot experiments on the competitive effects of charlock(Sinapis arvensisL.) on oats and wild oat(Avena fatuaL.) on barley had the following results:(1) During the period up to tillering there was an appreciable effect of the weeds on crop growth with depression in plant weight of 10–15% from that in the absence of weeds.(2) The greatest competitive effect occurred up to the middle of shooting of the cereals, when there were depressions in plant weight of 25–32%.(3) After this stage, the competitive effects of the weeds were much less in pots with a high nutrient level, and at low nutrient level there was almost no competitive effect.(4) The reduction in growth at the time of weeding was somewhat less at the low nutrient level than at the high nutrient level, and it is concluded that at the high level the competitive ability of the weeds was favoured more than that of the cereal crops.(5) The cereals were not able to overcome the initial damage caused by the weeds even when the weeds were removed at the beginning of tillering. A partial recovery did how ever occur at the high but not at the low nutrient level.(6) Both combinations, oats with charlock and barley with wild oat, behaved similarly, but some differences could be observed.Compétition entre plantes cultivées et mauvaises herbesII.Action des mauvaises herbes annuelles sur les c